1. Field of the Invention
The present invention relates to a transceiver, a modulation integrated circuit, and an RFID reader/writer, and particularly relates to a transceiver, a modulation integrated circuit, and an RFID reader/writer that enable improvement in S/N ratio (signal to noise ratio) during reception.
2. Description of Related Art
For example, JP-A-2004-206245 (patent literature 1) discloses an RFID (Radio Frequency Identification) reader/writer (noncontact IC card reader/writer) in the related art. FIG. 7 shows a block diagram showing a relevant part of the RFID reader/writer (hereinafter, abbreviated as “reader/writer”) 60 disclosed in the patent literature 1.
Referring to FIG. 7, a reader/writer 60 in the related art includes a radio transmission section 61, a radio reception section 71, and a circulator 66 leading a transmitted signal from the radio transmission section 61 to an antenna 67, and leading a received signal from an external tag (radio IC card) 50 received by the antenna 67, and performs transmission and reception with respect to the tag 50 via the antenna 67.
The radio transmission section 61 includes an oscillator 64 generating a carrier wave having a predetermined frequency, a modulator 62 modulating the carrier wave with an inputted transmitted baseband signal, a power amplifier 63, and a matching circuit 65. The radio reception section 71 includes a demodulator 72 in which a received baseband signal is generated.
Communication between the reader/writer 60 and the tag 50 is performed in the following way. In the case of transmitted data transmission, a carrier wave from the oscillator 64 is inputted into the modulator 62 which modulates the carrier wave with data (transmitted baseband signal). Then, such a modulated carrier wave is amplified by the power amplifier 63, and then transmitted from the antenna 67 via the circulator 66 through the matching circuit 65. In the case of only power transmission, the carrier wave from the oscillator 64 is transmitted with being not modulated. Transmission from the reader/writer 60 to the tag 50 is performed by interlinking of magnetic flux produced by the antenna 67 due to electromagnetic coupling with a not-shown antenna coil of the tag 50, exciting an induced voltage. In the tag 50, the induced voltage in the antenna coil is rectified by a rectifier circuit (not shown) in the tag 50 and then used for power of each of circuits in the tag 50. The same induced voltage is led to a demodulation circuit (not shown) to demodulate data from the reader/writer 60.
Next, the reader/writer 60 transmits a non-modulated carrier wave to perform only power supply to the tag 50 during data transmission from the tag 50 to the reader/writer 60. In a tag 50 side, a signal corresponding to data read from an incorporated memory (not shown) is outputted, which is received by the antenna 67, and demodulated by the demodulator 72.
FIG. 8 shows a view showing a transmission signal and the like between the reader/writer 60 and the tag 50 in the same time axis, after a command was sent from the reader/writer 60 to the tag 50. (A) in FIG. 8 shows a view showing command transmission by the reader/writer 60 and response reception timing to the command transmission, and (B) in FIG. 8 shows a view showing command reception by the tag 50 and response transmission timing to the command reception. (C) in FIG. 8 shows a view showing a waveform of a transmitted signal from the reader/writer 60 to the tag 50. (D) in FIG. 8 shows a view showing operation in the tag 50 and an aspect of change between reflection coefficients in response to response data, and (E) in FIG. 8 shows a view showing a response waveform of a transmitted signal from the tag 50 to the reader/writer 60.
Referring to FIG. 8, communication between the reader/writer 60 and the tag 50 is done in the following way. As shown in (A) in FIG. 8, after a command has been transmitted from the reader/writer 60 to the tag 50, a non-modulated wave is transmitted from the reader/writer 60 during response timing in the tag side ((C) in FIG. 8). As shown in (B), (D) and (E) in FIG. 8, response return in the tag side is transmitted by back-scattering the non-modulated wave. Here, back scattering means that a reflection coefficient A is changed into a reflection coefficient B by reflection from an antenna of the tag 50, in the tag 50. That is, in the tag 50 side, a reflection coefficient of the non-modulated wave transmitted from the reader/writer 60 is changed by the back scattering, thereby the response data are modulated ((E) in FIG. 8), and then returned to the reader/writer 60. The reader/writer 60 demodulates the returned signal by the demodulator 72 to obtain the response data from the tag 50.
Patent literature 1: JP-A-2004-206245 (paragraph number 0033, FIG. 2 and the like).